Roller-ball type computer mouses suffer from problems arising from the use of mechanical components. Optical computer mouses are readily available and overcome some of these problems. However, an optical computer mouse may suffer from other problems which hinder their acceptability.
One of these problems is the inability of currently available optical mouses to deal with the common use of lifting the mouse when there are space limitations. During movement of the mouse, it is common for a user to lift the mouse from the surface to move the cursor on the computer screen.
For example, if movement of the mouse is limited to 20 cm and this motion will move the cursor half-way across the computer screen, and if the user has already has moved the mouse to the right-most available position (i.e., limited by the mouse pad or by the space available on the desk), then the user will lift the mouse and move it to the left-most position possible. Afterwards, the mouse is replaced, and then moved again to the right so that the cursor will move the entire width of the screen.
This technique works for mechanical mouses since the ball does not rotate during lift-off and repositioning of the mouse. However, an optical mouse may see features during this procedure, leading to undesired cursor movement.
This problem has been addressed in U.S. Pat. No. 6,281,882 to Gordon, which discloses the use of a correlation product from the tracking algorithm of the mouse. Although this system will correctly identify lift-off, it will fail if the mouse is moving very quickly since the amount of correlation in the image will also be reduced by the motion.
U.S. Pat. No. 6,433,780 to Gordon discloses using a mechanical switch on the base of the mouse to detect lift-off. Although this will work, it adds to the complexity and cost of the mouse, and the additional mechanical features reduce reliability. The '780 patent also discloses the use of a special switch in which the user can press to suppress cursor movement. This adds an extra burden on the user and is undesirable. Even though the '780 patent mentions that the image will not be focused during lift-off, it does not disclose any method for detecting or using the loss of focus.